TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 7
7.3 Error Budgets
7.3.1 Dynamic and Static Error Budgets
Scope
Current work is focused on adding new modeling capabilities and validating those capabilities in
testbeds. Required models not presently in hand include: models of coating non-uniformities
across large optics, stray-light and scattered light models validated at 1 × 10 -11 fractional
scattered energy, and wave-front sensing and control models that demonstrate the ability to
identify speckles at the 1 × 10 -11 level.
Presently, there are no models of coating non-uniformities related to large-scale anisotropies in
the deposition process. These effects might limit the useful optical bandwidth. Stray light
models treat forward-scattered light as being uniformly shifted in phase relative to the nonscattered
beam. This approximation must be validated or superseded by new models. Stray-light
(multiply-reflected from baffles, edges, etc.) is calculated using standard stray-light software, but
the accuracy of the calculations at the 1 × 10 -11 level has never been validated.
Model validation for these new models and for existing ones (see below) is of the utmost
importance. The HCIT is used to verify/validate the models that are used to predict
requirements. These include diffraction, stray-light, scattered-light, polarization, mask
performance, and pointing. In contradistinction, the planned subsystem testbeds, namely the
sunshield, primary-mirror stability, secondary tower, and pointing-control testbeds, are used to
verify that dynamic requirements can be met. Unlike HCIT they do not verify models that are
used to predict the fundamental engineering requirements.
The schedule for the dynamic and static error budgets is given in Table 7-1.
State of the Art
TRL N/A
Static error budget terms are based on near-field propagation models, mask non-uniformity
models, polarization ray-trace, and bidirectional reflectance distribution function based
particulate-scattering models. The near-field propagation is calculated over the full bandwidth
(500-800 nm) and includes the power spectral density (PSD) profiles for surface phase and
amplitude. Mask design models evaluate complex transmission errors arising from design
limitation and manufacturing errors, leading to derived requirements on substrate transmission
PSD, lithographic/e-beam accuracy, and material dispersion. Ideal coronagraph mask
performance is based on Fourier-plane (image/pupil conjugate) computations that have a noise
floor better than contrast = 1 × 10 -13 . Polarization ray-traces are used to determine crosspolarization
level, the effectiveness of coating designs, and tolerances on those designs. Scatter
models assume that forward-scattered radiation dominates back-scattered radiation and is
coherent with the specular beam. At present there is no wavefront sensing and control model per
se. Instead, a WFSC performance limit is assumed, consistent with results obtained in the HCIT
testbed extrapolated to a more stable environment.
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